The Justice Department just gave the clearest sign yet that Mueller is homing in on Comey's firing

The Department of Justice won't allow the Senate Judiciary
Committee to interview two top FBI officials, Carl
Ghattasand James Rybicki,
about the circumstances surrounding former FBI Director James
Comey's dismissal in May, according to a letter obtained by CNN.

The DOJ cited "the appointment of Robert S. Mueller to
serve" as special counsel in charge of the Russia
investigation in a letter declining the committee's requests
for transcribed interviews with Ghattas and Rybicki.

"Under these circumstances and consistent with the
Department's long-standing policy regarding the confidentiality
and sensitivity of information relating to pending matters, the
Department cannot make Mr. Ghattas nor Mr. Rybicki available for
transcribed interviews at this time," the letter
read.

Ghattas is the executive assistant director of the FBI's
national security branch and Rybicki is the FBI director's
chief of staff and senior counselor.

That the DOJ cited Mueller and the Russia investigation when it
declined the committee's requests to interview Ghattas and
Rybicki indicated to legal experts that the FBI doesn't want its
own probe of the circumstances surrounding Comey's firing to be
compromised by Congress.

"It means that whatever the FBI officials would tell
Congress is relevant to what Mueller is investigating," said
former federal prosecutor Renato Mariotti. "It represents
official confirmation that the firing of James Comey and the
testimony of these two officials is part of the
investigation."

Mueller is also examining a
letter written by Trump outlining his reasons for firing
Comey that was heavily edited, and ultimately blocked, by White
House counsel Don McGahn.

Ty Cobb, the White House special counsel, disagreed with the
characterization of the president's letter as potentially
incriminating and called reporting to that effect "exaggerated
and/or fictionalized." But both McGahn and
then-White House chief of staff Reince Priebus
recently hired lawyers as the Russia investigation
continues, and Mueller will likely interview them to determine
whether Trump sought to obstruct justice and end the Russia
investigation when he fired Comey.

Mueller's questioning, Mariotti said, "will be most
effective if the witnesses know as little as possible about what
other witnesses will say" — which would be harder if FBI
officials gave an interview to Congress that would likely be
leaked and/or disclosed to Trump's defense team.

"The Congressional investigation serves an important
function, because it can investigate conduct that is not charged
as a crime," Mariotti said. "It can also reveal information to
the public about important events of public concern for which
there is a lack of evidence to charge. But those functions
typically take a back seat to ensuring that an ongoing criminal
investigation is not hindered in any way."